Note: Descriptions are shown in the official language in which they were submitted.
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~N~`OSS A/S, 6430 Nordborg, Denmark
~lectromagnetic ~low Meter
The invention'relates to an electromagnetic flow meter with a pressure
resistant ceramic measuring tube and a housing surrounding same to leave
a hollow space between them in which there are the pole shoes and coil
arrange;nent of a magnet system, sealing faces for the abutment of sealing
ring discs being provided at both ends,
In a ~nown flow meter of this kind (EU-OS 80 535), the measuring tube is
of an oxide ceramic so that it w.ill also be suitable for aggresive and
hot media. The measuring tube has such a high mechanical strength that
it is not necessary to-provide a metal covering which extends o~-er thc
entire length and increases the pressure and bending resistance (as in
the case of the construction of US-PS 37 50 46~)~Consequently, the pole
shoes and coil arrangement can be arranged directly beyond the ceramic
mPasuring tube and the metal housing can be used as the magnetic return
path. To,form an adequately large sealing surface, the metal tube has a
flange at both ends to resemble the shape of a yarn bobbin. The peripheral
faces of these flanges are he~d ir. a complementary bore Or the housing
and can be secured therein by means of shrinkage stresses. Ceramic
materials are highly sensitive to impact and percussion loads, pressure
peaks in the flowing medium, rapld temperature f`luctuatlons etc. ~f th?
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ceramic measuring tube breaks, the flow medium can escape over the entire
housing. This can cause extensive damage, especially in the case of
aggressive liquids.
The invention is based on the problem of providing an electromagnetic
flow meter of the aforementioned kind that is pressure tight despite
using a ceramic measuring tube.
This problem is solved according to the invention in that the housing
forms a pressure tight capsule of at least the same pressure resistance
as the measuring tube and its end faces form the outer part of the sealing
face.
In this construction, no measures are taken to strengthen the ceramic
measuring tube. Instead, the measuring tube is enclosed in a pressure
tight chamber. For this purpose, it is nece3sary on the one hand for the
housing to have an adequate pressure resistance and on the other hand not
only the measuring tube but also thc housing should be sealed at the ends
from connecting conduits. If the measuring tube should now break as a
result of exceptional stresses, the flow medium can simply flow into the
hollow .space between the measuring tube and the housing but not reach the
outside. Consequently even aggressive media cannot cause damage. Also,
the housing need generally have only a limited resistance to the flow
medium. This is because a break in the measuring tube is found relatively
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easily, for example through a change in the measuring -
signal, so th~t replacement of the flow meter is readily
possible within the service life of the housing material.
Especially suitable is our division of the housing into
a ceramic body, which covers almost half the flange, and
the remaining part of the outer housing is of a conductive
metal. One only needs to place a sealing ring of expanded
graphite which in itself is electrically conductive and which
extends so far inwards that it is in contact with the mediu~
flowing through the pipe. Hereby the same effect is
achieved as by the ground connections usually used where the
pipe conduit is made of or coated with isolating material.
Preferably, the housing has two flanges project-
ing inwardly up to the zone of the sealing face. This not
only gives the desired seal between the housing and con-
necting conduits but also leads to the measuring tube
having no, or no marked, outer flanges at its ends. This
considerably improves the production and strength of the
ceramic body.
Preferably, the housing is formed from a tube
and two flanges. These three partscan be readily sealing~
ly interconnected and provide a very strong housing. This
housing has a comparatively small external diameter so that
it can be readily accommodated within the clamping screws
that interconnect the connecting conduits and the spacing
of which from the central axis is standardised. In a pre-
ferred embodiment, the outer part of the sealing face and
the inner part of thesealing faceformed by the end face of
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the measuring tube have substantially the same radial ~Jidth.
As a result, the end of the housing provides a seal of the
same quality as the end of the measuring tube.
It is advisable for the inner part of the seal-
ing face formed by the end face of the measuring tube to
project somewhat axially beyond the outer
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part of tne sealing face. This results in the main seal during normal
operation being provided between the end of the measuring tube and the
connecting conduits.
Desirably, a circumferential seal is provided between the housing and
measuring tube at each end. It has been found that ceramic measuring
tubes break intermediate their ~nds so that the flow medium first penetrates
the hollow space of the housing. The circumferential seal i_ in series
with the outer sealing portion at the end so that the hollow space is
particularly tightly sealed.
Further, the electric terminals for the coil arrangement and for the
electrodes should be led out of the housing through pressure tight fittings,.
Another way of increasing the pressure tightness and possibly dispensing
with pressu~e tight fittings is for the free zone of the hollow space to
be rilled with a sealing mass.
A preferred example of the invention will now be described in more detail
with reference to the drawing in which the single figure sho~s a longltudinal
section through an electromagnetic flow meter according to the invention.
An elect.roma~netic flow meter 1 is clamped between the flanges 5 and 6 ol
two connecting conduits 7 and 8 with the interpositioning Or two sealing
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ring discs 3 and 4. Cla~ping bolts 9 with clamping nuts 10 and ll are
used for this purpose.
The flow meter comprises a ceramic measuring tube 12 with two weakly
formed flanges 13 and 14 of which the end faces constitute an inner
portion 15 and 16 of the sealing faces formed by the seaiing ring discs 3
and 4,
A housins 17 is formed by a tube 18 and two inwardly projccting flanæes
19 and 20. The ends of the housing define the outer part 20 and 21 of
the sealing faces formed by the sealing ring discs 3 and ~. The tube 18
has such a wall thickness and is so connected to the flanges 19 and 20
that the housing 17 has at least the same pressure resistance as the
ceramic measuring tube 12. The connection between the tube 18 and flanges
19 and 20 is effected by welding, adhesion or a pressure fit, possibl~ by
using additional sealing rings to result in a pressure tight ~oint.
Between the peripheral faces of the inwardly directed flanges 19 and 20
as well as the outer flanges 13 and 14 of the measuring tube 12 there is
a circumferential seal 23 and 24 in the form of two 0-rings. Alternatively,
there can be some other seal provided for example by putty, adhesive or a
pressure fit. A hollow space 25 remaining between the measuring tube 12
and housing 17 contains pole shoes 26 and 27 which abut the measuring
tube 12 from opposite sides, as well as associated coils 28 and 29. The
tube 18 of the housing forms the yoke, i.e. the magnetic return path Or
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the magnet system. The remaining free space of the hollow space 25 is
filled with a sealing mass 29 Two measuring electrodes, of which only
the electrode 31 is illustrated, are disposed ir. an axis which is perpendicular
to the plane of symmetry of the pole shoes 26 and 27 The leads to the
coils 28 and 29 and to the measuring electrodes extend through pressure
tight fittings of which the fittings 32 and 33 dispose~circumferentially
between the pole shoes 26 and 27 are illustrated in a position that is
offset from their actual position.
When clampine the flow meter 1 between the flanges 5 and 6, a seal is
produced not only in the region of the inner part 15 and 16 of the sealing
face but also in the zone of tne outer part 21 and 22 of the sealing
face, even though the inner sealing face forms the main seal because the
end face of the measuring tube 12 pro~ects somewhat, for example 1 mm,
axially beyond the end face of the housing 17. Should the ceramic measuring
tube 12 break for some reason, the flow medium can penetrate into the
hollow space 25. However, the housing 17 forms a tight capsule which is
sealed from the outside at both ends by the series disposition Or the
circumfere~tial seals 23, 24 and the outer part 20, 21 Or the sealing
face. For this reason, the flow medium in the hollow space 25 cAnnot
reach the outside. Even in the case of aggressive media, this protection
suffices to enable replacement Or the flow meter because the housing,
which maybe of steel, has an adequately long service life.
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As soon as the medium penetrates the hollow space 25, the magnet systemi., in~luenced. The measur~ng signal will change characteristically so
that the error can be recognised rapidly.
